molecules Article Neuroprotective Activities of Boophone haemanthoides (Amaryllidaceae) Extract and Its Chemical Constituents Abobaker S. Ibrakaw 1, Sylvester I. Omoruyi 2, Okobi E. Ekpo 3 and Ahmed A. Hussein 2,* 1 Department of Biodiversity and Conservation Biology, University of the Western Cape, Cape Town, Robert Sobukwe Road, Bellville 7535, South Africa; [email protected] 2 Department of Chemistry, Cape Peninsula University of Technology, Symphony Road, Bellville 7535, South Africa; [email protected] 3 Department of Medical Biosciences, University of the Western Cape, Cape Town, Robert Sobukwe Road, Bellville 7535, South Africa; [email protected] * Correspondence: [email protected]; Tel.: +27-21-959-6193; Fax: +27-21-959-3055 Academic Editors: Patrícia Rijo and Vera Isca Received: 11 October 2020; Accepted: 12 November 2020; Published: 17 November 2020 Abstract: Parkinson’s disease (PD) is a neurodegenerative condition that progresses as age increases, and some of its major symptoms include tremor and postural and movement-related difficulties. To date, the treatment of PD remains a challenge because available drugs only treat the symptoms of the disease or possess serious side effects. In light of this, new treatment options are needed; hence, this study investigates the neuroprotective effects of an organic Boophone haemanthoides extract (BHE) and its bioactive compounds using an in vitro model of PD involving the toxin 1-methyl-4-phenylpyridinium (MPP+) and SH-SY5Y neuroblastoma cells. A total of seven compounds were isolated from BHE, viz distichamine (1), 1α,3α-diacetylnerbowdine (2), hippadine (3), stigmast-4-ene-3,6-dione (4), cholest-4-en-3-one (5), tyrosol (6), and 3-hydroxy-1-(40-hydroxyphenyl)-1-propanone (7). Six compounds (1, 2, 4, 5, 6 and 7) were investigated, and five showed neuroprotection alongside the BHE. This study gives insight into the bioactivity of the non-alkaloidal constituents of Amaryllidaceae, since the isolated compounds and the BHE showed improved cell viability, increased ATP generation in the cells as well as inhibition of MPP+-induced apoptosis. Together, these findings support the claim that the Amaryllidaceae plant family could be a potential reserve of bioactive compounds for the discovery of neuroprotective agents. Keywords: Amaryllidaceae; Boophone haemanthoides; alkaloids; terpenoids; Parkinson’s disease; neuroprotection; apoptosis 1. Introduction Parkinson’s disease (PD) is a neurodegenerative disease that worsens with increasing age and affects about 10 million people worldwide. The initial manifestations of the disease occur at approximately 60 years of age, with females being less susceptible to the disease than males [1,2]. Although the incidence of PD has been strongly linked to age, a cross sectional study showed that approximately 30% of PD patients are younger than 65 years of age at the time of diagnosis [3]. The major symptoms of PD include tremor and postural and movement-related disorders [2]. These PD-related symptoms arise from the profound and selective loss of dopaminergic neurons in the substantia nigra pars compacta of the midbrain and the formation of Lewy bodies in the cytoplasm of neuronal cells. Although the etiology of PD is not fully understood, studies have shown that the loss of dopaminergic neurons can be associated with a number of factors, key among which are oxidative stress and mitochondrial dysfunction [4,5]. Biochemically, the earliest signs of PD involve the Molecules 2020, 25, 5376; doi:10.3390/molecules25225376 www.mdpi.com/journal/molecules Molecules 2020, 25, 5376 2 of 17 impairment of the mitochondrial electron chain impairment, alteration of mitochondrial dynamics, and an imbalance in calcium and iron homeostasis [6]. Following these changes, there is increased reactive oxygen species (ROS) generation in the mitochondria of neuronal cells, leading to a defect in the functioning of mitochondrial complex I, which is believed to be a major contributor to dopaminergic neuronal cell degeneration in PD [7,8]. In addition, the synthesis of adenosine triphosphate (ATP) is negatively affected, and the reduction of cellular ATP in turn drives dopaminergic neurons into programmed cell death (PCD) [9]. To understand the progression of PD in laboratory studies, the non-toxic chemical 1,2,3,6-methyl-phenyl-tetrahydropyridine (MPTP) is often used to model the disease in vivo [10,11]. Upon crossing the blood–brain barrier (BBB), MPTP is converted by the enzyme mono amine oxidase B (MAO-B) in astrocytes into the toxic metabolite 1-methyl-4-phenylpyridinium (MPP+), which leads to mitochondrial dysfunction [12]. MPP+ is known to cause the mitochondrial permeability transition pore to open, which in turn leads to a change in the mitochondrial membrane potential and increased ROS accumulation in the cells, which alters ATP levels and eventually induces apoptosis [6]. Although there is no specific cure for PD, levodopa, which is a dopamine-replacement therapy, is currently in use for the treatment of PD symptoms. However, the prolonged use of levodopa has been shown to be associated with some side effects, including the enhancement of oxidative stress and the acceleration of degeneration of residual dopaminergic neurons in PD patients using this medication [9], thus necessitating the search for alternative treatment options. A number of in vitro and in vivo studies have shown that herbal medicines, phytochemicals, and other plant-derived bioactive compounds and dietary supplements could ameliorate the effects of PD [13–15]. Boophone haemanthoides is a deciduous, winter-growing bulb plant that survives in almost all weather conditions, including the moist winter season, as well as in hot and dry summer temperatures. It belongs to the Amaryllidaceae plant family and is endemic to the winter and rainfall regions of South Africa and Namibia [16]. This plant family, which comprises over 800 species and 80 genera, is well distributed in the tropical regions of the world and found in abundance in the Southern Africa region of Africa [17,18]. Plants in this family are well known for their alkaloids, and so far more than 630 alkaloids have been isolated from these plants, many of which are known to possess a number of biological activities including antibacterial, anti-cancer, and neuroprotective activities [19–21]. B. haemanthoides was reported to be used traditionally by the Khoi-San tribe in the Northern Cape Province of South Africa for the treatment of asthma and for relieving knee pain [22]. Furthermore, a number of bioactive Amaryllidaceae alkaloids have been isolated from Boophone disticha, another member of the genus Boophone, including distichamine, buphanidrine, buphanisine, crinine, and distichaminol [23–25]. In one study, B. disticha was reported to show neuroprotective activities in 6-hydroxydopamine (6-OHDA)-induced SH-SY5Y toxicity by inhibiting ATP degeneration [26], while distichamine, buphanidrine, and buphanisine have also been reported to show a strong affinity for the neurotransmitter serotonin [20]. Importantly, the approval of galanthamine, an Amaryllidaceae alkaloid, for the treatment of Alzheimer’s disease by the US food and drug administration (FDA) has made alkaloids from this plant family potential sources of novel neuroprotective agents [27]. Thus, the present study investigates the neuroprotective activities of B. haemanthoides and its isolated compounds on MPP+-induced neuronal toxicity in SH-SY5Y neuroblastoma cells. 2. Results 2.1. Isolation and Identification of the Chemical Constituents B. haementhoids collected in South Africa were previously investigated, and eight alkaloids, including distichamine (1), were described [25]. Recently, triterpenes and other minor constituents including compounds 4–7 were described in our previous publication [28]. In this study we report on the isolation and identification of three additional known alkaloids, viz distichamine Molecules 2020, 25, 5376 3 of 17 (1), 1α,3α-diacetylnerbowdine (2), and hippadine (3) (Figure1), as well as the neuroprotection potential of compounds 1, 2, 4–7 against MPP+-induced toxicity in an in vitro PD model. Figure 1. Chemical structures of compounds 1–7 isolated from B. haementhoids. Compound 3 (hippadine) was isolated as minor constituent and identified according to its 1H-NMR and GC–MS analysis data, which was compared with available data in literature [29]. Compound 2 is described here as a natural product for the first time. It showed a typical positive reaction of alkaloids with Dragendorff reagent on TLC. The GC–MS analysis showed a single peak 1 at Rt 36.405 min, with m/z 403.4, corresponding to molecular formula C21H25NO7. H-NMR showed that an aromatic signal appeared at δH 6.11 (s,H10); two signals of methylenedioxy protons at 5.83, 5.79 d each (J = 1.3 Hz) (OCH2O); two proton signals of two methines appeared at 5.71 br t (J = 2.6 Hz) and 5.15 br quint (J = 2.6 Hz) of H1 and H3, respectively; two signals of a methylene group at 4.13, 3.76 d each (J = 17.3 Hz) and belong to C6; a proton signal at 3.48 dd (J = 5.5, 12.2 Hz); two acetoxy groups at 1.97 and 1.90; a methoxy at 3.95, in addition to signals of four methylene groups (C11,C12, 13 C2, and C4) (Table1). C-NMR showed 21 carbon signals classified using DEPT-135 and HSQC into a methylenedioxy carbon (δC 100.5), a quaternary carbon (δC 46.8, C10b), six aromatic carbons including a methine (δC 117.2, 140.4, 133.2, 148.4, 97.1), a methoxy (δC 59.1), six methylene (δC 28.6, 30.5, 57.5, 38.2, 51.0, 100.5 of C2,C4,C6,C11,C12, and OCH2O, respectively), and two oxygenated methines (δC 68.3 and 68.0), and two acetates (21.2/170.5, 21.3/170.1). Comparison of the given NMR data with literature indicated a crinane alkaloid with two acetoxy groups and a methoxy group, as shown in Figure1. Other 2D NMR experiments (COSY, HMBC, and NOESY) confirmed the structure and the positions of the two acetates and the methoxy groups.